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Frequently Asked Questions

What’s the difference between nuclear fission and nuclear fusion?
Both are a nuclear process, in that they involve nuclear forces to change the nucleus of atoms. On the contrary chemical processes involve mainly electromagnetic force to change only electronic structure of atoms. Basically, fission splits a heavy element (high atomic mass number) into fragments. On the contrary, fusion joins two light elements (low atomic mass number), forming a heavier element.  In both cases energy is freed because the mass of the remaining nucleus is smaller than masses of the reacting nuclei. The reason why opposite processes release energy can be understood by examining the binding energy per nucleon curve (see picture below).  High atomic mass elements and low atomic mass elements have a higher binding energy than mid-sized nuclei.  Both Fusion and Fission reactions shift the size of the reactant nuclei towards higher bounded nuclei.


 

Does Fusion produce radioactive nuclear waste the same way fission does?
Nuclear fission power plants have the disadvantage of generating unstable nuclei; some of these are radioactive for millions of years. Fusion on the other hand does not create any long-lived radioactive nuclear waste. A fusion reactor produces helium which is an inert gas. A fusion reactor does however produce and consume tritium inside the plant in a closed circuit. Tritium is radioactive (beta emitter) but its half life is short, furthermore it is used in a low amount so, unlike long-lived radioactive nuclei, it cannot lead to any serious danger. Safety handling of tritium is also much simpler. Regarding the activation structural materials in a fusion reactor it strongly depends on adopted solution for blanket and other structures, the reduction of activation is an important challenge for the next fusion experiments.

Can fusion cause a nuclear accident?
No, because fusion energy production is not based on a chain reaction like fission. Plasma must be kept at very high temperatures with the support of external heating systems and confined by external magnetic field. Every shift or change of the working configuration in the reactor causes the cooling of plasma or the loss of containment of plasma; in such a case the reactor automatically comes to a halt within a few seconds (i.e. the process of energy production is arrested), with no effects taking place outside it. For this reason fusion reactors are considered to be inherently safe.

Can fusion reactors be used to produce weapons?
No. Although hydrogen bombs do use fusion reactions, they require an additional fission bomb in order to detonate. Magnetically-confined fusion cannot alone produce the amount of instantaneous power required for a weapon.

What are the effects of fusion on the environment?
Fusion is among the most environmentally friendly sources of energy. There are no CO2 or other harmful atmospheric emissions from the fusion process, (i.e. fusion does not contribute to greenhouse gas emission or global warming). Its two sources of fuel, hydrogen and lithium, are widely available in many parts of the Earth.

When is electricity generated through fusion expected to be available?
Performance of a fusion device is usually measured by a characteristic calculation called the Triple Product. The picture below shows the values of the Triple Product in different magnetic confinement fusion devices from the sevenths to the end of the century, for comparison, the developments of computer chips following the moor law are also indicated. Progress in fusion research has been very fast, as fast as progress in computational power in those years. The expected value of Triple Product for ITER is also reported, however ITER plasma operations are expected in the early twenties.

 

At present, fusion devices produce more than ten megawatts of fusion power, and ITER will be capable of producing 500 megawatts of fusion power. Although it will be on the scale needed for a power station, there will still be technological issues to address in order to produce steady state operations. A prototype of a Fusion Reactors (DEMO) is expected to be built in less than 30 years. Electricity generation is expected to take place in 30 to 40 years, depending on funding and technical advancement.

What temperatures are reached in fusion experiments?
In order to obtain a thermonuclear fusion reactor with a positive energy balance it is necessary to bring the deuterium and tritium plasma to about 100 million degrees (the equivalent of about 10 KeV of kinetic energy), six times the temperature inside the sun.

How is the hot plasma contained in a tokomak device?
At a temperature of 100 million degrees it is very difficult to contain the plasma inside a container. However plasma is made up of charged particles, and it is therefore possible to use magnetic fields to guide the particles. In a tokamak the plasma is contained inside a toroidal vacuum chamber, a strong magnetic field with a specific configuration generated by external coils and the plasma current, which prevents the plasma from coming into contact with the walls of the chamber.

IAEA Book on Fusion Physic
 
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Responsible/Contact: Physics Section | Last update: 18 September 2014